We will apply a 32P-labeling technique to investigate age- dependent DNA modifications, termed I-compounds, as potential biomarkers of aging. I-compounds are recently discovered DNA adduct-like DNA derivatives that were found in our laboratory to increase with age in rodent tissues. I-compound profiles are strongly dependent on tissue, species, and (to a lesser extent) sex of animals. They appear to show little inter-animal variation, both qualitatively and quantitatively, in animals of the same age. I-compounds may thus provide a quantitative biomarker of aging that would be useful in the biomonitoring of humans if measured in easily accessible cells or tissues. We will test this hypothesis using rats of the NIA/NCTR biomarker research colony, focusing on DNA of white blood cells and skin, i.e. tissues available from humans with a minimum degree of invasiveness. White blood cell and skin DNA will be isolated from individual female and male Fischer 344 (F344), Brown-Norway (BN), and F344 x BN hybrid rats of different ages, from both ad libitum fed and dietary restricted groups. There will be 8 age points: 4, 6, 8, 10, 12, 16, 20, and 24 months and 6 rats per point. In addition, liver and kidney DNA will be isolated from pooled tissue samples of the biomarker research colony rats at each of the age points. Levels and patterns of I-compounds will be analyzed by a 32P- postlabeling assay for covalent DNA modifications. A total of 1536 DNA specimens will be examined over the 5-year course of the project. Analysis of liver and kidney DNA of the rats will show whether I-compound changes in internal organs parallel those in white blood cells and skin. At each age, a sufficient number of analyses will be conducted to assess this approach in terms of its reproducibility, sensitivity, significance of rate of change, non-lethality (e.g., use of white blood cells), potential relevance to human aging, and technical requirements. This research will provide a definitive answer to the question as to whether the measurement of I-compounds will provide a useful non-invasive method for biomonitoring of aging.